Radio frequency micro-electromechanical system (RF MEMS) switches are basic building blocks for construction of various RF components and sub-systems such as variable capacitors, phase shifters, tunable RF matching circuits/filters, and reconfigurable antennas, to name a few. RF MEMS switches can be classified in two general groups based on their electrical contact methods: 1) metal-to-metal ohmic (or DC) contact switches, known as resistive series switches, and 2) capacitive contact switches, known as capacitive shunt switches. The resistive series switches typically have a small up-state capacitance, e.g., on the order of 2-8 femptofarads (fF), that exhibits excellent isolation from DC to 20-40 Gigahertz (GHz), and also a small down-state contact resistance of 1-2 ohms (Ω) that results in a low insertion loss of 0.1-0.2 decibels (dB). However, because resistive switches rely on multiple metallic surfaces in contact with each other during current passage, microwelding and stiction become significant drawbacks of resistive switch implementation.
To address these and other drawbacks, capacitive shunt switches were developed using a coplanar waveguide (CPW) topology, in which a metallic membrane of the switch is in contact with an insulative dielectric layer (typically SiNx or SiO2) that covers the signal electrode of the conductive line. The shunt capacitive switch typically has an up-state capacitance of 20-70 fF and low signal line resistance of 0.1Ω that results in a low insertion loss. When the capacitive switch is closed, it has a down-state capacitance of 2-5 picofarads (pF) yielding an almost ideal RF short to ground, thereby providing high isolation only at relatively high radio frequencies (e.g., above 15 GHz).
Although capacitive RF MEMS switches have advantages over resistive switches in the amelioration of microwelding and stiction issues, the isolation of capacitive switches tends to be fair (20 dB) at 10-25 GHz and poor (˜10 dB) below 10 GHz. This poor isolation at frequencies below 10 GHz precludes capacitive switches from being employed for most commercial applications. Various designs to improve the isolation of capacitive switches in the down-state have been proposed. For instance, some designs, referred to as inductively tuned X-Band switches, have added an additional inductive conductive-line section to the standard design in order to achieve an 8 dB improvement in isolation. However, these X-Band switches only achieve improved isolation in the 10+ to 12 GHz frequency range because it becomes very difficult to further lower the resonant frequency to C-band frequencies due to the impractically large inductance required. To sufficiently increase the inductance requires a shape change of conductive line and brings in discontinuity for the waveguide, which requires difficult design compensations.
Accordingly, it would be desirable to provide capacitive RF MEMS switches capable of achieving higher isolation across low frequency ranges.